The present invention generally relates to an impeller or vane wheel and more particularly, to a mixed flow impeller for a blower which is widely employed in a domestic or industrial air conditioner, ventilating fan or the like.
In recent years, the mixed flow impeller has been broadly used for various products such as air blasting arrangements for air conditioning, heating appliances, and cooling of electronic equipment, ventilating fans, etc. However, noises produced by the mixed flow impeller as it rotates through air present a main problem related to these products, which are frequently used in places closely related to life environment, and reduction of noises has been strongly required recently, also from the aspect of elimination of public nuisance by noises with respect to the neighborhood.
Conventionally, as shown in FIGS. 1 and 2, a propeller fan having a simplified guide includes an impeller 3 constituted by a cylindrical hub 1 and a plurality of blades or vanes 2 each having a generally uniform thickness and secured to the outer peripheral portion of said hub 1, and a guide member 6 disposed around the peripheral portion of said impeller 3 so as to partition a suction section 4 from a delivery section 5 as illustrated.
Noises by such a fan as referred to above and arising from an aerodynamic cause may be broadly classified into the following two kinds, one of which is a discrete frequency noise showing a peak value at some frequencies to be determined by the number of blades and rotational speed, and the other of which is a broad-band frequency noise which shows a gentle spectrum distribution with respect to the frequency. The former noise is produced by interference between the blades and surrounding solid walls or periodical turbulence, while the latter noise is mainly attributable to variation in a lift resulting from discharge of turbulent vortex from the trailing edge of the blade or generation of blade tip vortex, etc.
In order to reduce such noises, various counter-measures have been proposed up to the present. In the arrangement as in the propeller fan described with reference to FIG. 2(a), an effect mainly for reducing the discrete frequency noise can be obtained by providing the blade face with a tilting angle .theta. in the rotating direction. Meanwhile, through employment of various blade profiles, for example, of NACA (National Advisory Committee for Aeronautics), it has been possible to suppress separation of air flow from the blade surface, thereby to reduce generation of turbulent noises, with a simultaneous improvement of air moving performance.
However, the tilting of the blade face in the rotating direction brings about inconveniences as described hereinbelow.
As important items for determining aerodynamic performance of an impeller, there may be raised an entrance angle .beta.1, and exit angle .beta.2 and a chord L.sub.C, etc. as shown in FIG. 2(b). Now, it is assumed that the fundamental blade shape which satisfies the target performance has been determined as shown in FIG. 3. Here, on the supposition that the relation .theta.=.theta..sub.0 provides the optimum tilting angle for reducing noises, the blade shape may be designed as shown in FIG. 4 by tilting the blade face in FIG. 3 by .theta..sub.0. Upon comparison of a cross section along the line Va-Va in FIG. 3 with that along the line Vb--Vb in FIG. 4, the blade of FIG. 4 shows the shape more inclined in the blasting direction as seen from FIG. 5. Such an inclination angle is determined by the tilting angle and the shape of the original impeller, and if this angle is altered, the air flow in a radial direction is affected, with consequent variation in the aerodynamic performance of the impeller. Corrections are required in order to prevent such a disadvantage, but this may undesirably complicate the designing of the impeller, thus extremely obstructing proper selection of aerodynamic performance and tilting angle at the optimum values.
Moreover, in the thin blade impeller made of sheet metal or the like, it is impossible to adopt the blade profiles of NACA, etc., and even in the case of blades made of resin, there are problems related to weight increase due to larger blade thickness, insufficient strength, cost increase, sinkage or cracking during molding, etc., and thus, blade profile impellers have not be employed except for a particular case.